The present invention relates to a machine package having a sound absorbing structure for reducing noise radiated from an opening provided for cooling down heat generated from a machine, such as an industrial machine, having a suction port and an exhaust port.
Most typically used as a conventional sound absorbing structure for an opening are: a lined duct using a porous material such as glass wool, a sprit type, a cell-type, etc., a basic form of each of which is a duct lined with a sound absorbing member.
The duct lined with a sound absorbing member faces a decrease in the amount of sound reduction in a high sound area where the wavelength of sound is smaller than the diameter or short side of a cross section thereof since a sound wave travels in a beam-like form. Often used to prevent this defect as much as possible are: a cell type as a parallel type of thin straight paths formed by dividing the duct cross section in a grid form by a sound absorbing member; and a splitter type sound absorbing duct dividing the flow path in parallel by a tabular sound absorbing member.
However, even with these types, the amount of sound reduction is controlled by sound absorbing properties of the sound absorbing member and the length of the duct subjected to sound absorbing processing. Thus, to provide effect for high sounds and further increase the sound absorption coefficient in a low sound area by providing the split type, the cell type, or the like, it is required to increase the thickness of the sound absorbing member, thus resulting in an increase in the fluid resistance. The conventional sound absorbing structure of a sound absorbing duct type requires space for noise in a band of 500 to 2 kHz which finds the widest applications, and thus faces problems concerned with costs, weight, etc. and also a problem of antinomy that an attempt to enhance the noise reduction performance increases the airflow resistance and then deteriorates the cooling performance.
Additionally, it is also possible to achieve noise reduction by installing a louver or forming the duct into a maze shape, although it suffers from the same problems as described above.
As their solution, Japanese Patent Application Laid-Open Publication No. H9-126666 describes a sound reducing assembly having substantially circular-cylindrical sound absorbing members formed of a sound absorbing material and also arranged in at least two rows across an air inlet.
Japanese Patent Application Laid-Open Publication No. 2000-87725 describes an acoustic damping material formed with a sound absorbing member and a acoustic reflection member provided on one side of this sound absorbing member and having a cross-sectionally concave-shaped reflection surface so that sound transmitted through and incident on the sound absorbing member is absorbed while being reflected by the reflection surface to elongate the sound absorbing distance in the sound absorbing member and then emitted to the side where sound S has arrived.
Japanese Patent Application Laid-Open Publication No. H9-26177 describes an air duct having a sound absorbing function that, by fitting a sound absorbing member using ion exchange fiber to a gas flow path, utilizes sound absorbing effect and gas pollutant removing operation to purify gas. Japanese Patent Application Laid-Open Publication No. 2002-266756 describes a sound absorber which has, inserted in a rectangular-cylindrical casing, a circular-cylindrical sound absorbing element having a pipe of an inorganic fiber whose front and rear surfaces are coated with an anti-scattering material of breathable inorganic fiber, organic fiber, glass cloth, nonwoven fabric, or the like.
The conventional duct lined with a sound absorbing member or, as its application, the cell-type and the splitter-type have many problems in practical aspects such as sound reducing performance, space, weight, costs, etc., since an attempt to increase the amount of sound reduction for a band of 500 to 2 kHz in highest need of sound reduction requires narrowing down the duct length, the thickness of the lined sound absorbing member, and the opening, which results in an increase in the airflow resistance.
The configuration described in Japanese Patent Application Laid-Open Publications No. H9-126666 and 2000-87725 has the cylindrical sound absorbing member arranged in such a manner as to intersect with airflow, and thus provides the effect of reducing the airflow resistance, but did not give sufficient consideration to sound absorbing properties concerning the material of the sound absorbing member with respect to the sound absorbing effect.
Further, the configuration described in Japanese Patent Application Laid-Open Publications No. H9-26177 and 2002-266756 has the sound absorbing member arranged in parallel to airflow and thus has the same problem as the aforementioned cell-type and splitter-type have, and further does not give sufficient consideration to sound absorbing properties concerning the material of the sound absorbing member with respect to the sound absorbing effect.